1. Field of the Invention
The present invention relates to the field of mode-switching transformers which are used to convert common-mode signals into differential signals and conversely. Such transformers are generally called “baluns”, for balanced-unbalanced.
The present invention more specifically relates to the forming of a balun having a one quarter or 1 (common mode) to 4 (differential mode) impedance ratio, noted 1/4, by using a thin layer technology in electronic circuit applications.
The present invention, for example, applies to the field of radio frequency transmissions and of mobile telephony.
2. Discussion of the Related Art
FIG. 1 illustrates the terminals of a balun 1 of the type to which the present invention applies. Balun 1 comprises a first terminal 2 intended to receive or to provide a common-mode signal UNBAL and two differential-mode accesses 3 and 4 intended to provide or to receive differential signals BAL1 and BAL2. A voltage Vc applied on the common-mode side (for example, a signal referenced to ground and originating from an antenna) is converted into two differential voltages V1 and V2 on differential-mode terminals 3 and 4, referenced to ground 5 (identical or not to ground) of the electronic circuit.
Baluns can be divided in two categories: baluns with local elements formed of inductive and capacitive elements associated in a specific electric assembly and baluns with coupled lines formed of conductive lines forming inductive elements of a transformer. The present invention applies to the second category, that is, to mode-switching transformers in a coupled-line technology.
FIG. 2 shows the electric diagram of a conventional balun with coupled lines having an impedance ratio different from one (for example, 1/4).
Such a balun with a 1/4 ratio is intended for circuits in which the impedance on the differential side (block 13, Zdiff1 connecting access 3 to ground and block 14 Zdiff2 connecting access 4 to ground) is four times greater than the impedance (block 12, Zin) on the common-mode side. In fact, with impedance values Zdiff1 and Zdiff2 (for example, 100 ohms), each of which is equal to twice impedance Zin (for example, 50 ohms), a 1/4 balun is needed (50/200) for the circuit to be impedance-matched without requiring additional adapters.
A mode-switching transformer with coupled lines comprises, on the primary side (common mode), a conductive line 23 defining an inductance L3 and, on the secondary side (differential mode), two conductive lines in series defining inductances L1 and L2 between terminals 3 and 4, junction point 6 between lines 21 and 22 being connected to ground 5. Lines 21 and 22 are coupled to line 23, which is twice as long as each line 21 or 22. The line lengths are a function of the work frequencies of the balun. Generally, lines 21 and 22 have lengths corresponding to one quarter of the wavelength of the central frequency of the bandwidth desired for the balun, line 23 having a length corresponding to half this wavelength.
The impedance ratio of the balun is set by the value of a capacitor C connecting the free end of line 23 to ground 5. For example, for a 1/4 ratio and for a balun sized for a central 2-GHz frequency, a capacitor on the order of 1 pF is required.
A disadvantage is the presence of the capacitor which needs to be adjusted according to the desired impedance ratio.
Another disadvantage of the use of a capacitor to match the impedance ratio is that this makes the mode-switching transformer sensitive to electrostatic discharges, above all in an integrated forming.
Another disadvantage is the capacitor bulk.
Another disadvantage is the complexity of the forming.